This invention relates to an apparatus and method for drying electrophoresis gels following the process of electrophoresis separation.
Gel electrophoresis is a common procedure for the separation of biological molecules, such as DNA, RNA, polypeptides and proteins, in which the molecules are separated into bands according to their migration through a filtering gel in response to an imposed electric field. The extent of migration is a function of a number of factors, including the charge density of the migrating molecules and their molecular weight, size and shape.
The basic apparatus used in this technique consists of the gel, usually formed from polyacrylamide, agarose or starch, enclosed in a glass tube or sandwiched as a slab between glass or plastic plates. Electrophoresis gels generally have an open molecular network structure, with spaces large enough to admit passage of the migrating molecules, placed in a buffered solution of a salt that conducts an electric current in water solution. These gels are typically 70% to 95% water.
After the various migrating molecules have been separated within a slab gel, it is common to dry the gel prior to conducting analysis of the bands of the migrating molecules within the gel.
While drying, electrophoresis gels tend to shrink in any dimension that is not supported by filter paper, film or other support media. Substantial shrinkage of the gel tends to make analysis of the migration bands difficult, if not impossible. Uneven gel surfaces resulting from uncontrolled shrinkage also tend to make storage of the dried gel difficult. In order to be useful, the dried gel should be flat and substantially maintain its length and width. These characteristics are best maintained by supporting the surfaces of the gel with filter paper, film, or other support media while drying.
Three attributes generally are required of a gel drying system to get flat, intact dried slab gels. First, the gel must adhere to a porous supporting material such as a cellophane membrane or filter paper throughout the drying process. This support maintains the length and width of the gel during drying. Should part of the gel detach from the support during drying, the gel may crack; air bubbles between the gel and the support tend to prevent attachment. Second, the supporting materials, in turn, must be secured against a surface or around the perimeter so that the support remains flat and dimensionally stable. Should the support slip or move during the drying process, the gel may crack or distort. Finally, all portions of the gel should dry uniformly. If one area dries faster than an adjacent area, uneven stress develops, often leading to cracking. These three attributes are required in an effective drying system to prevent gel cracking.
A variety of methods and apparatus have been used to facilitate the drying of electrophoresis gels. One such method, disclosed in U.S. Pat. No. 4,883,597, consists of extracting water vapor from a gel supported by a hydrophobic polyethylene membrane by applying a vacuum through the membrane. The vacuum also holds the gel firmly against the membrane.
Other more complex gel drying devices utilize both heat and vacuum sources to vaporize moisture from the gels. The combination of the heat and vacuum is often used in an attempt to uniformly remove moisture from the gels so as to avoid distortion or cracking of the gel during the drying process. Such a drying procedure commonly entails positioning the gel matrix on a filter paper and gel matrix within a drying apparatus in which the combination is subject to a vacuum source and heated to remove liquid from the gel. One example of such a device is disclosed in U.S. Pat. No. 4,020,563.
Similarly, U.S. Pat. Nos. 4,612,710 and 4,788,778 disclose a method and apparatus in which heat is supplied to a horizontal gel slab from a heating plate above the slab while drawing a vacuum beneath.
The drying of electrophoresis slab gels without vacuum is known. A relatively simple, inexpensive device for drying at ambient pressure is the Mini-Gel Dryer.TM. drying system manufactured by Novex (Novel Experimental Technology) of San Diego, Calif. That device, however, allows for vaporization to occur on only one side of the gel. The stresses caused by drying on only one side of the gel may lead to cracking.
A vacuum-less drying method which allows evaporation from both sides of a slab gel is described in K. Wallevik and J. C. Jensenius, J. of Biochemical and Biophysical Methods, 6 (1982) 17-21. The method is accomplished using a device having two identical frames held together by metal binder clamps. Between the frames is a sandwich of a slab gel between two sheets of uncoated cellophane. When properly assembled, this device securely holds the cellophane supporting sheets, and the gel adheres to the cellophane. In order to obtain good results with this technique, the gels must be flooded with liquid; however, no provision is made for containing the potentially hazardous, excess liquid. Applicants overcome this disadvantage through the addition of a trough to the drying system of applicants' invention for the accumulation of excess liquids.
Hoeffer Scientific Instruments manufactures a gel drying system consisting of a platform that fits inside half of the frame; this frame-platform combination makes a level surface, simplifying assembly of the gel-cellophane sandwich. Kem-En-Tec manufactures a different system, in which legs built into the frame suspend the frame assembly horizontally above the bench top. Unlike applicants' invention, the systems manufactured by Hoeffer and Kem-En-Tec make no provision for containing potentially hazardous, excess liquids.
In addition, the drying systems manufactured by Hoeffer and Kem-En-Tec use two different frame sections. The frame sections interlock to avoid any horizontal sliding during assembly. Because the frame sections are not identical, two different mold cavities must be fabricated. Applicants' invention overcomes this disadvantage by utilizing identical frame sections that require only a single mold cavity.
Furthermore, all of these prior art techniques hold the frame assembly horizontally to dry the gel, which often results in evaporation from the top surface being faster than from the bottom surface. Such uneven evaporation may lead to cracking or other distortion of the gel. This problem may be overcome by directing an air stream over the top and bottom surface of the dryer.
In view of the disadvantages of the prior art methods and apparatus that have been used to dry electrophoresis gels, an apparatus is needed for drying electrophoresis slab gels in ambient air without distortion or fracture and utilizing a method which is easy to perform without complex steps or complicated equipment.